It is important that such units be small and unobtrusive and highly reliable. It is also desirable that there should be certain operational options available, which could be factory-set or made field selectable.
One particular problem is premature triggering of an alarm. With a delicately set detector sensitive to vibrations, for example, a single shake such as may be occasioned by a passing lorry rattling a window may set it off. If an intruder was attempting entry, the disturbance would be more prolonged and repeated. It is therefore desirable to distinguish between the two types of disturbance.
It is also useful to know, when investigating a disturbance to know which of a group of detectors was excited first, but without a complex wiring arrangement back to the central control.
While the invention is designed primarily to be used in conjunction with a vibration sensitive element, there is no reason why its principles should not be applied to the processing of a disturbance signal generated by other means, such as interference of a light beam or contact with a pressure pad.